1. Field of the Invention
The present invention relates to a hologram recording method and a hologram recording apparatus, and specifically, to a hologram recording method and a hologram recording apparatus capable of suppressing the distribution of a Fourier spectrum, preventing the deterioration in S/N ratio, and obtaining high diffraction efficiency.
2. Brief Description of the Related Art
In a holographic storage for recording data in a recording medium by a hologram, an object beam (signal beam) is Fourier transformed with a lens and is recorded. This holographic storage has advantages that it can record data with high density and it is tolerant of defects and misalignment of recording media and optical systems.
On the other hand, in the intensity distribution of a Fourier transformed image of the object beam, since a DC (direct current) component, i.e., zero-order component is often stronger than a high-frequency component and the DC component exceeds the dynamic range of the recording medium, it has been difficult to record high quality images. Especially, an analog image such as a face image contains many DC components as low-frequency components in image parts represented by cheeks and forehead, and the ratio of DC component to high-frequency component often becomes a four-digit number or more. Accordingly, it becomes difficult to linearly record a hologram in all Fourier spectrum regions, and thus, deterioration in S/N ratio and reduction in diffraction efficiency occur.
In order to solve the above described problems, suppression of DC components of the object beam is effective. In the digital holographic storage, recording is performed in a defocus position of the signal beam and a uniform Fourier transformed image is obtained using a random phase mask or pseudorandom phase mask (see, for example, Holographic Data Storage Springer pp. 259-269 and Japanese Patent Application Laid Open (JP-A) No. 2004-198816).
However, there are problems that DC components can not be suppressed sufficiently by the method of recording in a defocus position of the signal beam, and the alignment accuracy of the phase mask is required in the method of using the phase mask.